The present invention relates to programmable crystal oscillators. In particular, the present invention is directed toward a programmable crystal oscillator having an adjustable capacitive load circuit coupled to the crystal.
Crystal oscillators are widely used to generate timing signals for electronic hardware, such as computers, instrumentation, and telecommunications equipment. Crystal oscillators typically include a quartz crystal and an oscillator circuit, which electrically excites the crystal so as to generate an oscillating signal at a resonant frequency determined by physical characteristics of the crystal. The oscillator circuit or a separate output circuit (buffer) wave-shapes the oscillating signal into a timing pulse train acceptable to the electronic hardware.
Timing frequencies are specified by the electronic hardware manufacturers and thus vary over a wide frequency range. However, a crystal's resonant frequency is determined by its physical characteristics, e.g., size, shape, crystalline structure, etc. Trimming the crystal's resonant frequency can be achieved by selective metal plating the crystal faces. Consequently, the manufacture of crystal oscillators is an involved process that is both time consuming and costly. Thus, suppliers of crystal oscillators stock large numbers of crystal oscillators manufactured to a variety of standard output frequencies. However, if a customer requires a custom frequency, a manufacturer generally must "start from scratch" by dicing an ingot into crystal wafers of specific dimensions and then subjecting the crystal wafers to numerous processing steps (lapping, etching, and plating), all designed to achieve the custom output frequency. Custom crystal oscillators come at premium prices and require long manufacturing leadtimes (months).
Since virtually all crystals are capable of oscillating, manufacturing yield is quite high. However, if the crystal's resonant frequency cannot be trimmed to meet one customer's frequency specification, the crystals are typically inventoried in the hope that they can be used to meet another customer's frequency specification. In the case of custom crystal oscillators, it is not uncommon for manufacturers to produce an oversupply of custom crystals to ensure a sufficient volume of crystals capable of meeting customer requirements in terms of both output frequency and quantity. The excess crystal oscillators are then placed in inventory. Maintaining large crystal inventories represents a significant manufacturing expense.